The council of Constantinople held in 754 was a significant council. It considered itself to be ecumenical
“The holy and Ecumenical synod, which by the grace of God and most pious command of the God-beloved and orthodox Emperors, Constantine and Leo,  now assembled in the imperial residence city, in the temple of the holy and inviolate Mother of God and Virgin Mary, surnamed in Blachernae, have decreed as follows….” (Percival, Henry R (2013-06-23). The Seven Ecumenical Councils (p. 681). Veritatis Splendor Publications. Kindle Edition.)
What does it mean that it was ecumenical? For a council to be considered ecumenical is to be understood that this council is speaking for all of Christendom. It is not a local governing body that intends to make statements only of a particular place or for a moment in time. It makes decisions that are to reflect the course of church history.
The Council of Constantinople is significant because it considered itself to be ecumenical and then as an ecumenical council determined/decreed:
(1) If anyone shall not confess, according to the tradition of the Apostles and Fathers, in the Father, the Son and the Holy Ghost one godhead, nature and substance, will and operation, virtue and dominion, kingdom and power in three subsistences, that is in their most glorious Persons, let him be anathema.
(2) If anyone does not confess that one of the Trinity was made flesh, let him be anathema.
(3) If anyone does not confess that the holy Virgin is truly the Mother of God, etc.
(4) If anyone does not confess one Christ both God and man, etc.
(5) If anyone does not confess that the flesh of the Lord is life-giving because it is the flesh of the Word of God, etc.
(6) If anyone does not confess two natures in Christ, etc.
(7) If anyone does not confess that Christ is seated with God the Father in body and soul, and so will come to judge, and that he will remain God forever without any grossness, etc.
(8) If anyone ventures to represent the divine image (charakter) of the Word after the Incarnation with material colours, let him be anathema!
(9) If anyone ventures to represent in human figures , by means of material colours, by reason of the incarnation, the substance or person (ousia or hypostasis) of the Word, which cannot be depicted, and does not rather confess that even after the Incarnation he [i.e., the Word] cannot be depicted, let him be anathema!
(10) If anyone ventures to represent the hypostatic union of the two natures in a picture, and calls it Christ, and thus falsely represents a union of the two natures, etc.!
(11) If anyone separates the flesh united with the person of the Word from it, and endeavours to represent it separately in a picture, etc.!
(12) If anyone separates the one Christ into two persons, and endeavours to represent Him who was born of the Virgin separately, and thus accepts only a relative (schetike) union of the natures, etc.
(13) If anyone represents in a picture the flesh deified by its union with the Word, and thus separates it from the Godhead, etc.
(14) If anyone endeavours to represent by material colours, God the Word as a mere man, who, although bearing the form of God, yet has assumed the form of a servant in his own person, and thus endeavours to separate him from his inseparable Godhead, so that he thereby introduces a quaternity into the Holy Trinity, etc.
(15) If anyone shall not confess the holy ever-virgin Mary, truly and properly the Mother of God, to be higher than every creature whether visible or invisible, and does not with sincere faith seek her intercessions as of one having confidence in her access to our God, since she bare him, etc.
(16) If anyone shall endeavour to represent the forms of the Saints in lifeless pictures with material colours which are of no value (for this notion is vain and introduced by the devil), and does not rather represent their virtues as living images in himself, etc.
(17) If anyone denies the profit of the invocation of Saints, etc.
(18) If anyone denies the resurrection of the dead, and the judgment, and the condign retribution to everyone, endless torment and endless bliss, etc.
(19) If anyone does not accept this our Holy and Ecumenical Seventh Synod, let him be anathema from the Father (Percival, Henry R (2013-06-23). The Seven Ecumenical Councils (p. 684). Veritatis Splendor Publications. Kindle Edition.)
Today many iconodules (Greek eikono-doulos “One who serves images”) reject this council as ever being an ecumenical council. The reasons are another discussion. But my chief concern is how much weight we place in the historical councils. In A.D. 787 another council would meet that called itself the Seventh Ecumenical Council of Nicea that pronounced unanimously that the worship of images “is agreeable to Scripture and reason, to the Fathers and councils of the Church.”
If the council was ecumenical or not is not relevant to me. So that I’m clear, I do think the councils are very important. However, even councils and heroes of the faith need to be judged supremely by the Holy Spirit speaking in the Scripture.
Westminster Confession of Faith 1.10 reads:
The supreme judge by which all controversies of religion are to be determined, and all decrees of councils, opinions of ancient writers, doctrines of men, and private spirits, are to be examined; and in whose sentence we are to rest; can be no other but the Holy Spirit speaking in the Scripture.(a)
(a) Matt. 22:29, 31; Eph. 2:20 with Acts 28:25.
Opponents of Sola Scriptura speak against Protestants for the many denominations or possible interpretations. But when we look at the alternative, the tension is not resolved either. Councils didn’t agree. Church Fathers had to be interpreted. There are even many schools of interpretation of Thomas Aquinas. Thus we must hold to Scripture as our ultimate authority.